EXPERIMENTS AT ROTHAMSTED 



97 



the barley and oat experiments, a considerable supply of 

 nitrogen from the soil made available to the crop by spring 

 and summer tillage. 



By adding 400 Ibs. of ammonia salts to the superphosphate 

 and alkali salts we obtain a further increase of 64 cwts. of 

 potatoes, or a return of over 16 cwts. for i cwt. of sulphate of 

 ammonia employed. This is a highly profitable result. The 

 crop obtained on this plot (plot 7) exceeds by more than 2 

 tons the crop given by 14 tons of farmyard manure on 

 plot 3. The farmyard manure plot received superphos- 

 phate as well as dung during the first seven years of the 

 experiment. 



The need of a full supply of ash constituents if ammonia 

 salts are to be used with profit is most strikingly shown by the 

 crop on plot 5, which receives ammonia salts without ash con- 

 stituents ; the produce here scarcely exceeds that of the per- 

 manently unmanured land. A result of this kind is of course 

 due to the exhaustion of the available potash and phosphates 

 in the soil during the preceding experiments with wheat. 



Influence of Climate. The return from the ammonia 

 salts is still better in a fine season. In the next table will be 

 found the average produce of the plots already mentioned in the 

 years 1878 and 1881. The potatoes grown in 1878 were the 

 Rock, in 1881 the Champion. 



TABLE XXXII. 



AVERAGE PRODUCE OF POTATOES VARIOUSLY MANURED AT 

 ROTHAMSTED, SEASONS 1871 AND 1881. 



